1. Field of the Invention
The present invention relates to transmission antenna diversity, and more particularly, to a transmission antenna diversity method, and a base station apparatus and a mobile station apparatus therefor in a mobile communication system.
2. Description of the Related Art
Third generation mobile communication systems have standards for transmitting data at a higher rate than second generation mobile communication systems represented by personal communication systems (PCS). In Europe and Japan, a synchronous Wideband Code Division Multiple Access (W-CDMA) mode is adopted as a wireless access standard. In North America, an asynchronous CDMA-2000 mode is adopted as a wireless access standard. Mobile communication systems are configured so that many mobile stations can communicate through a single base station.
It is necessary to overcome fading in order to transmit data at a high rate in a mobile communication system. Fading reduces the amplitude of a received signal by several decibels to several tens of decibels. A variety of diversity techniques are used to satisfactorily overcome fading. In a CDMA mode, a delay spread of a channel is employed. A reception diversity technique of receiving a multi-path signal is applied to a Rake receiver. This reception diversity technique has a problem in that reception diversity does not operate well when delay spread is small. A time diversity technique using interleaving and coding is used in a Doppler spread channel. It is difficult to use this method in a low-speed Doppler channel.
Space diversity is used to overcome fading in an indoor channel having small delay spread and an outdoor channel which is a low-speed Doppler channel. Space diversity uses two or more antennas. In this method, when a signal transmitted through one antenna is reduced due to fading, a signal transmitted through another antenna is used for reception. Space diversity is divided into reception antenna diversity using receiving antennas and transmission antenna diversity using transmitting antennas. It is difficult to install reception antenna diversity in a mobile station because of lack of space and excessive cost, so it is recommended to use transmission antenna diversity in a base station.
Conventional transmission antenna diversity can be a closed loop transmission, wherein up-link channel information is fed back from a mobile station or it can be an open loop transmission diversity, wherein feedback is not sent from a mobile station. When using L antennas, closed loop transmission diversity has L times greater gain than open loop transmission diversity in terms of a Signal to Interference and Noise Ratio (SINR). However, the performance of closed loop transmission diversity of feeding back channel information for operation is influenced by the period available for feedback. When the feedback period is long, a channel may change before feedback information reaches a base station, thereby degrading the performance. Further, when a large amount of information is fed back per unit time in order to track a rapidly changing channel, the up-link capacity decreases.
In addition, transmission antenna diversity is divided into a maximal ratio combining (MRC) method, an equal gain combining (EGC) method, and a selective combining (SC) method. When a feedback bandwidth is not satisfactorily secured, the performance of the above-described closed loop transmission antenna diversity may be degraded because a change in channel information is not reliably reflected in the feedback information signal. Thus, in order to have channel information that is rapidly and reliably reflected in the feedback information signal, closed loop transmission antenna diversity employing an SC method is typically used.
However, when using an SC method, unbalance between antennas can occur, causing additional costs to configure a radio frequency (RF) processor. Therefore, diversity using an SC method for overcoming the above problem and performing diversity using less feedback information is desired.
Although diversity gain can be obtained, a SINR gain increase causes an attendant decrease in diversity when using an SC method compared to diversity using an MRC method or an EGC method because channel information is not completely indicated in the feedback information. Therefore, an improved diversity method is desired that can maximize SINR gain by compensating for the decrease, can be applied at a high moving speed, and can simplify the hardware configuration of a transceiver.
A transmission antenna diversity method using a feedback mode is disclosed in U.S. Pat. Nos. 5,634,199 and 5,471,647. In these patents, measurement of a channel and a feedback method using a perturbation algorithm and a gain matrix are proposed. However, these patents adopt a blind method, which is slow to converge in measuring a channel and is not effective in finding an exact weight. Thus, the method is not frequently used in systems that use a pilot signal.
In Universal Mobile Telecommunication Service (UMTS) W-CDMA (3GPP) standards, Motorola proposes a method of quantizing a weight for each antenna in a feedback mode. In addition, Nokia and others propose a transmission antenna diversity method for high-speed mobile objects operating with respect to two antennas. However, these methods are optimized for a case of using two antennas. Therefore, an improved method of effectively selecting many antennas is desired.